Many techniques and arrangements for making prestressed bodies of formable materials such as concrete, plaster or ceramics, for example, are known today, yet none of them is capable of effective implementation without considerable difficulty in construction. Unless the bodies are correctly prestressed, they will be subject to considerable internal stresses and cracking.
One well-known technique for manufacturing bodies and structures incorporates a prestressing arrangement within the bodies themselves. According to this technique, prestressing tendons are temporarily anchored in place and pulled tight within a body being formed before fixing or hardening the formable material over the prestressing tendons. Once the formable material hardens, the prestressing tendons are released from their anchors. The resulting tensile forces in the prestressing tendons establish compressive forces in the body, preloading the body against tensile forces.
It is, however, often difficult to prestress the tendons tightly enough. This is particularly the case in constructing complex bodies or structures, because the tendons of the arrangement cannot be pulled taut except in straight lines. Accordingly, conventional techniques are generally limited to use in the manufacture of elongated or prismatic bodies or structures.
Attempts have been made to solve this problem by heating the tendons and anchoring the hot tendons in a hardened body, then allowing the tendons to cool and contract to prestress the body. However, this technique has its limitations and is unsatisfactory for many reasons. In particular, the large amount of heat required to prestress the tendons adequately in large bodies makes the technique highly impracticable in many instances. U.S. Pat. No. 2,414,011, issued to K. P. Billner in 1942, illustrates this technique and shows some of the shortcomings and difficulties involved in prior techniques.
It is well known that certain structural materials known as shape memory materials change in form, shape and length not only due to usual thermal expansion and contraction, but also according to so-called shape memory effects. Such materials in effect "remember" a certain original shape they held at a prior time, and when subjected to heating above a particular threshold or transformation temperature level, they return to this original shape. In certain metallic alloys, this shape memory effect manifests itself in a diffusionless, solid-state transformation, which is known as martensitic phase transformation. Nickel-titanium and certain copper-based alloys are examples of materials which can undergo martensitic transformation.
When such alloys are subject to a well-known thermomechanical process, the martensitic phase structure is produced. The structure may be deformed substantially from its original shape. However, it promptly changes back to its original shape, when its temperature is increased beyond a particular material-characteristic temperature threshold level or point. This threshold level is frequently referred to as the transition temperature. Upon reaching this temperature, the martensitic metal structure "remembers" its original shape and tends to return thereto.
As is well known, the memory effects of martensitic alloys have been commercially used in several applications, including use in tube couplings. According to this particular commercial application, the "original" shape of the tube coupling has a diameter which matches the dimensions of a pair of adjacent tube ends to be joined together. The tube coupling element is then deformed by a well-known process to a larger diameter, which permits the coupling to be slipped over the tube ends to be joined. After being slipped over the tube ends, the coupling is heated beyond its threshold temperature, and it consequently returns toward its original smaller diameter. A very strong bond is thus produced between the tube ends joined by operation of the shape memory effect.
The change in size produced in the shape memory material as a result of transformation can be substantial. For example, certain known alloys of nickel-titanium will reduce in size by as much as 8%) Material bodies made of a particular brass alloy are known to reduce in size up to 4%. As is also well known, a substantial number of shape memory alloys are commercially available which have transition temperatures ranging from 20.degree. C. to 150.degree. C.